I agree with everything you said except for your last statement. Microsoft has stated multiple times the 8.1 upgrade is free for everyone with windows 8.
There really is not a lot that is different. As far as UI changes go, on the desktop, there is a button to bring you to the metro interface (that they call a "start button").
Also, the metro interface has a way to browse all applications now. Those are the only noticeable UI changes I've seen.
Under the hood, they've added a 3D printing driver interface.
So, really, there is not much to worry about and the available preview version should be adequate for testing. I still wish we could test on the final version however.